1. Field of the Invention
This invention relates, in general, to pressure-sintered polycrystalline mixed materials with a base of hexagonal boron nitride, oxides and carbides. In one aspect, this invention relates to mixed materials having a density of at least 94% of the theoretical density. In a further aspect, the invention is directed to pressure-sintered polycrystalline mixed materials which are particularly useful for detachment or break rings in the horizontal continuous casting of steel and non-ferrous metals.
2. Description of the Related Art
In the horizontal continuous casting of steel, the mold is connected in an airtight manner to a trough. The solidification of the steel, i.e., the formation of the strand shell in the mold, does not, therefore, take place, as in all other continuous casting systems at a phase boundary between liquid steel and the ambient gas, but in a continuous liquid phase.
The junction between trough and mold is formed by a furnace brick which is securely fixed in the furnace and on the mold side by a discharge nozzle which is composed of high-grade refractory material as, for example, zirconium oxide, since it has to absorb high thermal stresses. Adjacent to this discharge nozzle is the detachment or break ring, which is a characteristic component of horizontal continuous casting. This detachment ring has to insure, in continuous operation, that the formation of the strand shell starts from this point on. It, consequently, serves to fix the position of solidification precisely in the mold. Since it is unavoidable that a shell also forms at the detachment ring, the former has to be readily detachable, and must not form an interphase with the material of the detachment ring. In addition, a high thermal shock resistance and high resistance to abrasion are required.
In the systems for the horizontal continuous casting of steel developed since 1960, detachment rings of ceramic material such as silicon nitride, zirconium oxide, boron nitride or aluminum nitride have become known. It also became known, however, that if, for example, silicon nitride rings were used, it was not possible to prevent defects typical of horizontal continuous casting such as transverse cracks at the surface on pulling the solidified ingot out of the mold.
Refractory mixed materials with an oxide base are particularly durable because of the addition of graphite, and have, therefore, proved particularly suitable for immersion nozzles in conventional continuous processes (cf. DE-A-2,936,480, DE-A-3,003,046 and DE-A-3,627,337). These materials cannot, however, be used for break rings in horizontal continuous casting because they interfere with the casting of high-alloy special steels, which are particularly sensitive to the absorption of carbon.
Refractory materials of sintered stabilized zirconium oxide, having a boron nitride content not exceeding 20% (cf. JP 50-21,575, C.A. 100: 196759e, 1984), however, fail to meet the requirements imposed to the same extent as those of zirconium oxide having a boron nitride content of up to 70%, which were produced using mixtures of granulated and powdered boron nitride and are, accordingly, porous (porosity at least 15%) and are, consequently, only of low density (cf. JP 78-45,312, C.A. 89: 94066p, 1978) since chipping and erosion at the break ring are unavoidable. These are associated not only with increased wear of the break rings themselves, but also with undesirable inclusions in the steel which result in a marked reduction in quality and, in particular, with the long casting times required for high-alloy special steels.
Materials of boron nitride alone or with a boron nitride base, in which the hexagonal boron nitride fraction is 80 to 60% by weight and the oxide fraction is composed of zirconium oxide or magnesium oxide, having an adequate density of at least 94% of the theoretical density (based on the boron nitride/oxide mixture), have the advantage over the other known materials that steel and steel alloys virtually do not wet them and in addition, they are easy to work, which is of decisive importance for achieving a break ring shape which is a precise fit at the point of use.
Break rings manufactured from hexagonal boron nitride are, however, too soft and have unduly low mechanical strength with the result that they have a tendency to chipping and erosion, even when low-alloy steels are being cast. Provided that the materials have been sintered to a density of at least 94% of the theoretical density, oxide mixtures, and in particular, ZrO.sub.2 or MgO admixtures, do, in fact, achieve improved properties (cf. DE-A-3,510,111) which do not, however, satisfy the requirements imposed in relation to mechanical and chemical resistance and, in particular, for casting high-alloy special steels such as Cr-Ni steels.
Consequently, there is a need for improving the material with a base of boron nitrides or oxides by selecting suitable admixtures in a manner that meets the requirements imposed for use as a detachment or break ring in the horizontal continuous casting, not only of steel, but also of non-ferrous metals.
According to DIN 17 600, non-ferrous metals are understood to mean unalloyed metals, with the exception of iron, and alloys in which any metal, except iron, represent the largest single content.